The present invention relates to determining the range of moving targets, and more particularly, to a method of passively determining the range of moving targets from a single test platform which is either stationary, or moving at a constant velocity along a linear trajectory.
Various passive ranging systems and methods have been previously developed. In a passive ranging system the position of a target, such as an aircraft or ship, is typically determined from radiant signals emanating from the target itself, rather than echo signals developed from radiant energy being transmitted towards the target, as in active ranging systems. An advantage of a passive system is that it precludes the detection of the active tracking stations by unfriendly forces during hostilities. Generally, the systems and methods previously developed involve the use of a variety of optical, electromagnetic and acoustic sensors, as well as the use of cooperating ground based receiving and transmitting stations. Many such systems also require a plurality of cooperating sensor units, and cross-correlation equipment, as disclosed in U.S. Pat. No. 4,060,809. Such systems are generally complex, and often include operating limitations such as the ability to sense only continuously emitted signals, or the inability to be effective against targets moving at a velocity greater than the velocity of the observing vehicle itself.
Many of the problems associated with prior passive ranging systems and methods have been overcome by the passive ranging methods described in U.S. Pat. No. 4,179,697 (the '697 patent), issued Dec. 18, 1979 in the name of the present inventor, Martin Golinsky, and in U.S. Pat. No. 4,558,323, issued Dec. 10, 1985, also in the name of the present inventor. Both the '697 patent and the '323 patent disclose passive ranging methods which permit the range to an emitter target to be determined from a single moving vehicle, commonly referred to as a test platform.
In the method disclosed in the '697 patent, a single measuring aircraft is constrained to fly along a curved trajectory at a constant speed, while simultaneously performing a sequence of passive bearing measurements of the target aircraft relative to itself. Such measurements produce a geometric pattern of intersecting rays which provides the data for mathematical calculations used to determine the target's range.
In the method disclosed in the '323 patent, a single measuring aircraft is again used, however, it is constrained to fly along a linear trajectory at a non-constant speed (i.e., under acceleration or deceleration), while also simultaneously performing a sequence of passive bearing measurements of the target aircraft relative to itself. Again, such measurements are used to produce a geometric pattern of intersecting rays which also provides the data for a mathematical solution to the target's range.
Although both of these methods are valid and useful for certain applications, there are other applications where it would be desirable to passively measure the range of a target emitter without a test platform having to meet the travel constraints imposed by these prior methods. For example, it is desirable for any aircraft traveling in a normal manner, i.e., traveling along a linear trajectory at a constant velocity, to be able to readily measure the range to other aircraft without going off its assigned course or changing its normal air speed. Similarly, it is also desirable for a stationary test platform, such as an air traffic control tower, or a hovering helicopter, which are incapable of satisfying the travel constraints of the prior methods, to also be able to measure the range to other aircraft.